Overuse injuries to the lower extremity are of major concern to both participants and researchers. O f all injuries to the lower extremity, the knee is one of the most frequently injured sites. The purpose of the study was to investigate mechanical and muscular mechanisms of lower extremity joints during landing from three different heights (0.32 m, 0.62 m and 1.03 m) using three landing techniques (soft, normal, and stiff). Biomechanical models were employed to study muscle activity, energy transfer and dissipation, and joint responses. Nine active male subjects volunteered to participate in the study (1 8 -3 5 yrs). Two video cameras (200 Hz), two force platforms and electromyography (1000 Hz) were used to obtain right sagittal kinematic, force and muscle activity data, respectively.
The lower extremity joint ranges of motion decreased as the landings became stiffer and increased as the landing height increased. Significantly greater first knee maximum moments and minimum powers were observed, indicating the greatest mechanical demands on the knee joint during the initial impact. The knee and hip extensors absorbed more energy than the ankle plantarflexors in the soft landing . For the stiffer landing, the lower extremity musculature demonstrated a decreased ability to dissipate energy with the hip extensors' ability being most dramatically reduced while the ankle plantarflexors demonstrated an increased ability to contribute in the energy dissipation.
The results exhibited high magnitudes for the first and second maximums of the patellar tendon, quadriceps femoris tendon and patellofemoral compressive forces. The maximum forces experienced at the patellar and quadriceps tendons were below rupture values assessed in the literature. The data also indicated energy transport via the biarticular muscles from distal to proximal during landing, due to the sequential occurrences of the second minimum powers of the gastrocnemius and rectus femoris muscles. Finally, vertical stiffness values were used to characterize the spring characteristics of the body during landings. High first and low second vertical stiffness maximums were observed approximately at the times of the first and second maximum ground reaction forces